In recent years, semiconductor devices such as memory devices and processors have been used for a variety of electronic devices such as personal computers, tablet terminals, and smartphones. The semiconductor devices are being improved in many aspects such as low power consumption and miniaturization.
One of the ways that have been proposed to achieve lower power consumption, miniaturization, and the like is the use of an oxide semiconductor for a semiconductor layer (hereinafter referred to as an active layer, a channel layer, or a channel formation region in some cases) of a transistor in a semiconductor device. For example, a transistor in which an oxide of indium, gallium, and zinc (hereinafter referred to as an In—Ga—Zn oxide in some cases) is used for a channel layer is given (see Patent Document 1).
Examples of the semiconductor device for which a transistor including an oxide in a channel formation region is used include a memory device and a processor. The transistor has a feature of extremely low off-state current flowing between a source and a drain (hereinafter referred to as a leakage current in some cases), and a memory device including the transistor can retain memory without power supply (see Patent Document 2). Furthermore, when the transistor is used for a processor, high-speed backup and high-speed restore can be achieved and the processor can have reduced power consumption (see Non-Patent Document 1).